A fuel cell comprises an electrolyte layer and a pair of catalyst carrying electrodes placed on either side of the electrolyte layer, and generates electricity through an electrochemical reaction between fuel fluid such as hydrogen or alcohol and oxidizing fluid such as oxygen or air, which are supplied to the corresponding electrodes, with the aid of the catalyst. There are a number of different types of fuel cells that have been proposed. Many of them use liquid electrolytes, but those using solid electrolytes are being preferred more and more for the ease of fabrication and handling.
However, the voltage output produced from each of such solid electrolyte type fuel cells is very low, typically in the order of 1 volt or less, and most applications require substantially higher voltages. Therefore, it is necessary to connect individual fuel cells electrically in series. Typically, such series connection of fuel cells is achieved by stacking the fuel cells to form a fuel cell stack, but it has been also proposed to provide a fuel cell assembly in the form of a sheet in that a plurality of fuel cells are arranged in a common plane. (International Publication WO01/95406).
In this fuel cell assembly, a pair of separators (or flow distribution plates) interposing an electrolyte layer therebetween and defining passages for fuel gas (e.g., hydrogen) and oxidizing gas (e.g., oxygen) are each formed with a plurality of recesses, which serve as gas flow passages, corresponding to a plurality of fuel cells in a matrix pattern such that adjacent recesses are connected to different gas supply systems and thus adjacent fuel cells have opposite polarities. Each fuel cell can be connected to its adjacent fuel cell by an associated one of gas diffusion electrodes each formed of, e.g., a carbon sheet comprising a platinum (Pt) catalyst and disposed on either side of the electrolyte layer so as to face the gas flow passages (or the recesses of the separators) such that the fuel cells are connected in series as a whole. The gas diffusion electrodes are made of a porous material and thus tend to have a large electric resistance. For this reason, it has been also proposed to achieve the cell-to-cell connection by electroconductive films formed on a surface of each separator facing the electrolyte layer by vapor depositing gold, for example, to thereby reduce the electric resistance.
Component parts such as the separators for defining a plurality of fuel cells can be preferably manufactured by using a semiconductor process or micromachine process such as etching a substrate consisting of a single crystal silicon or glass. Particularly, the separators for use in a small fuel cell assembly comprising fuel cells having a power of about 1-100 W and used in place of a battery or the like require a high level of precision that can be hardly attained by machining, but it is possible to manufacture such separators with sufficiently high precision and high efficiency by using the semiconductor process or micromachine process.
Thus, according to the above proposition, a fuel cell assembly in the form of a sheet (or a planar fuel cell assembly) and having a plurality of interconnected fuel cells can be achieved. In such a fuel cell assembly, however, the gas diffusion electrodes or electroconductive films for connecting the cells are disposed between the electrolyte layer and each separator, and thus it is practically impossible to change the cell connection pattern once the fuel cell assembly has been assembled.
Even when a fuel cell assembly comprises only a single fuel cell, it may be sometimes desirable that an outer side of each separator is provided with an electrode for connection to external devices. For example, such a configuration can make it easier to stack a plurality of fuel cell assemblies and connect them in series to form a fuel cell stack. FIG. 14 shows an example of such a fuel cell assembly that allows an electrode to be provided on the outer side of each separator. This fuel cell assembly 100 comprises a pair of separators 111, 112 each made of silicon, for example, and formed with a recess 110 for defining a flow passage for a fuel fluid (e.g., hydrogen gas) or an oxidizing fluid (e.g., oxygen gas), an electrolyte layer 113 interposed between the pair of separators 111, 112, and a pair of diffusion electrodes 114 disposed on either side of the electrolyte layer 113 so as to face the recesses 110 of the separators 111, 112. Each diffusion electrode 114 comprises a catalyst electrode layer 115 contacting the electrolyte layer 113, and a diffusion layer 116 adjoining the recess 110 of the separators 111, 112. In this fuel cell assembly 100, the surface of each separator 111, 112 is coated with an electroconductive film 120 formed by vapor deposition, for example, so that an electric potential of each diffusion electrode 114 can be transmitted from an inner surface of each separator 111, 112 contacting the diffusion electrode 114 to an opposite outer surface of the same via the electroconductive film 120, thereby making it possible to provide the outer surface of the separators 111, 112 with an electrode 121 for connection to external devises. However, in such a fuel cell assembly 100, the electroconductive path implemented by the electroconductive film 120 tends to be long and result in an undesirably high internal resistance of the fuel cell assembly 100. Further, although it may be relatively easy to deposit the electroconductive film 120 evenly on the top and under surfaces of the separators 111, 112, it is difficult to deposit the electroconductive film 120 evenly on side surfaces of the separators 111, 112 which are perpendicular to the top or under surface. This can result in an undesirably thin electroconductive film 120 formed on the side surfaces and thus increase the internal resistance of the fuel cell assembly 100.
Japanese Patent Application Laid-Open (kokai) No. 2000-173629 has disclosed to set a plurality of metallic pin or a metallic plate having a plurality of protrusions in an insert molding die, and inject molten resin material therein to form an integrally molded separator such that the metallic pins or the metallic plate extends through the separator. In this way, electric potential of the electrode (anode or cathode) contacting the inner surface of the separator can be transmitted to the outer surface of the separator via the metallic pins or the metallic plate. Since the electroconductive path thus implemented extends through the separator instead of covering it, the electroconductive path can be shorter and result in a smaller internal resistance of the fuel cell assembly. However, such an approach cannot be applied to a separator formed by etching a substrate made of an inorganic material such as single crystal silicon, glass or the like.